A sharp increase in backbone network data traffic imposes a significantly higher requirement for a transmission rate of a fiber communications system. However, an optical communications system has a limited hardware bandwidth. To transmit a large amount of data in the limited bandwidth, it is essential to improve frequency band utilization.
The foregoing problem can be effectively resolved by using a modulation scheme of high frequency band utilization. Quadrature amplitude modulation (QAM) is a combination of a quadrature-carrier modulation technology and multi-level amplitude keying, features relatively high frequency band utilization, and is widely applied to fields such as optical communications, wideband wireless communications, and digital cable television transmission. A square 16-QAM modulation scheme used in an existing optical communications system features high costs, large power consumption, and high system complexity, and is hard to implement.